In conventional photography, silver halide particles are used to capture incident radiation and form a latent image. The latent image is later amplified in a development step. However, silver halide is only sensitive to ultraviolet and blue radiation. Therefore, to capture images in color the silver halide must be made sensitive to other wavelengths of visible light. This is normally accomplished by adding a spectral sensitizing dye. The dye can be added at different times during the preparation of the silver halide emulsion, chemical sensitization, or spectral sensitization process. The dye can be added in many different ways, as a solution in water or an organic solvent, as a gelatin dispersion, or as a suspension of microcrystalline particles. It is frequently necessary to use a combination of more than one dye to adjust the wavelength where the emulsion absorbs the maximum amount of light, or to control some other property of the emulsion. This is particularly true for J-aggregating cyanine dyes (where several dye molecules interact in such a way as to cause a shift in the absorption of light to longer wavelengths with a very narrow absorption band). J-aggregating dyes are preferred in many cases for their narrow absorption and high extinction.
When using more than one dye, the timing of the addition of the dyes can be very important. Also, the order in which the dyes are added often makes a difference in the final distribution of light-absorbing molecules on the emulsion surface. With J-aggregating cyanine dyes, the most desirable state in most three-color photographic applications is to have the two dyes intimately mixed on the surface so that the mixture absorbs light as a single aggregate, not like the two individual dyes. This can sometimes be very difficult to achieve if the dyes have a tendency to strongly adsorb as separate J-aggregates.
The common practice in manufacturing is to add the two dyes separately with a waiting period in between to allow the dye to equilibrate onto the surface of the silver halide particles. This method often leads to variability when the emulsion is prepared in different batch sizes, because the addition time and mixing are varying. Another concern in the practice of spectral sensitization of photographic silver halide emulsions is to use methods in manufacture that are efficient and environmentally benign. The use of organic solvents in manufacture is undesirable because the solvents can be released to the atmosphere as they evaporate out of the coated film. Aqueous dispersions with or without gelatin are preferred because they do not use volatile solvents.